Spring (March–May) and autumn (September–November) are the two clear windows. Spring brings rhododendron blooms and clear mountain views in the early weeks before the haze of pre-monsoon builds up; autumn is famously stable, with cold-clear days and the most reliable views of Chomolhari from Chele La and Drukgyal Dzong. Both seasons coincide with the major festivals.
The Paro Tsechu, held in the courtyard of Rinpung Dzong over five days in late March or early April, is the country's most-attended festival. Masked cham dances re-enact the deeds of Guru Rinpoche and various protector deities; the final morning unfurls a giant thondrol scroll before dawn. Book accommodation many months ahead if you are timing a trip around it.
Monsoon (June–August) brings clouds that often obscure the mountains, daily rain, and leeches on the Tiger's Nest trail. Winter (December–February) is cold but bright, and the lower-altitude sights remain very visitable; the high passes including Chele La can close after heavy snow.